1. Field of the Invention
The present invention relates to a semiconductor device fabricated using a III-V nitride semiconductor material and specifically to a semiconductor device including an n-type nitride semiconductor layer grown over a p-type nitride semiconductor layer and a fabrication method thereof.
2. Description of the Prior Art
As a light source for high-density optical discs, a violet-range laser light source, which is capable of emitting light in a short wavelength range to form a converged light spot of a small diameter on optical discs as compared with red-range light and infrared light and therefore effective in improving the recording density of optical discs, has been demanded.
Presently, to realize violet-range laser light, a semiconductor laser element fabricated using a III-V nitride semiconductor, such as gallium nitride (GaN), or the like, has been developed.
FIG. 10 is a cross-sectional view of a conventional nitride semiconductor laser element. The semiconductor laser element shown in FIG. 10 includes an n-type GaN layer 102, an n-type cladding layer 103 of n-type AlGaN, an n-type guide layer 104 of n-type GaN, a MQW active layer 105 of InGaN, an overflow suppressing layer 106 of p-type AlGaN, a p-type guide layer 107 of p-type GaN, an n-type current confinement layer 108 of n-type AlGaN which has an opening 108a serving as a current waveguide, a p-type guide layer 109 of p-type GaN, a p-type cladding layer 110 of p-type AlGaN, and a p-type contact layer 111 of p-type GaN, which are sequentially epitaxially grown over a substrate 101 of n-type GaN.
An electric current injected to the laser element does not flow in the n-type current confinement layer 108 but flows through the opening 108a formed in the n-type current confinement layer 108. Light generated in the MQW active layer 105 is confined by the difference between the refractive index of the n-type current confinement layer 108 and the refractive index of the p-type guide layer 109 which is greater than that of the n-type current confinement layer 108. The current confining structure by a pn junction and a light confining structure by a hetero junction are integrally referred to as a buried structure.
To form the above-described buried structure, generally, magnesium (Mg) at a concentration of about 5×1018 cm−3 to 20×1018 cm−3 is added as a p-type dopant to the p-type semiconductor layers, e.g., the p-type guide layer 107, and silicon (Si) at a concentration of about 1×1018 cm−3 to 4×1018 cm−3 is added as an n-type dopant to the n-type semiconductor layers, e.g., the n-type current confinement layer 108.
However, the above-described conventional buried-type semiconductor laser element has the following problems.
As described above, magnesium (Mg) at a concentration of about 5×1018 cm−3 to 20×1018 cm−3 is added to the p-type overflow suppressing layer 106 and the p-type guide layer 107. Further, silicon (Si) at a concentration of about 1×1018 cm−3 to 4×1018 cm−3 is added to the n-type current confinement layer 108 formed over the p-type guide layer 107.
In general, the n-type current confinement layer 108 having such a structure is considered to have a current confinement function. However, in some actually-fabricated buried-type laser elements, the n-type current confinement layer 108 fails to perform the current confinement function, an insufficiency of the current confinement function disables the element to work as a laser element, or the laser characteristics are deteriorated.